One "person of interest" is injured and in custody, police commander Kerry Yamaguchi told reporters, following the incident at the King Soopers store in Boulder, some 30 miles northwest of the state capital Denver.
Live-streamed video earlier had showed a white middle-aged man -- shirtless and seemingly covered in blood -- detained by police and led from the supermarket. "My heart is breaking as we watch this unspeakable event unfold in our Boulder community," said Colorado governor Jared Polis, calling the incident a "senseless tragedy."
"It felt amazing that everybody was helping each other out and that our instincts were on the same page and we ran... I don't know why other people didn't, and I'm sorry that they froze. I wish that this just didn't happen."